This invention is directed to improved systems for providing motive force for vessels using sails, and more particularly is directed to a sail system and method which permits development of substantial driving power at close angles to the wind.
Sails have been utilized to harness the power of the wind since ancient times. A prime desire has been to design systems that permit sailing at as small an angle to the wind as possible while still maintaining a sufficient forward force to drive the vessel in a forward direction. It has been difficult to design sailing rigs to accomplish that objective, partially because of the many factors which must be taken into account, including the necessary ability to handle large variations in wind velocity, the amount of heeling force which is generated by the rig, the necessity of providing a sail plan which can sail efficiently at other angles to the wind, the spatial constraints imposed by the necessity of fitting a rig to a vessel that can move through the water efficiently, and many other factors.
Most modern sailing craft, particularly the smaller pleasure craft, utilize a sloop rig comprising a jib, and a mainsail mounted on a main mast, possibly with one or more other masts bearing sails, usually farther aft of the main mast. Much of the review of the aerodynamics of sailing has been centered on the sloop rig, with emphasis on the interaction between the jib and the mainsail, and the way the jib affects the air flow over the mainsail. As used herein, the term "jib" refers to a triangular sail projecting forward of a mast, the leading edge (or luff) of which is not directly attached to the mast along its length, but is supported by a line or stay or similar thin support, which in turn is supported by the vessel and the mast, and typically runs between the deck and the mast. The jib, like all sails, acts not only as an object for the force of the wind to act upon, but also effects the direction of the wind which acts upon it. Most previous rigs have generally been designed with the idea of forming a perfect slot between the jib and the mainsail, so that the jib draws the air smoothly past the leeward side of the mainsail without backwinding the mainsail. In practice, even the most highly tuned jib and mainsail systems of racing yachts generally require a minimum angle of about 22.degree.-25.degree. from the apparent wind (usually about 40.degree.-45.degree. from the true wind), with forward force falling off drastically if closer angles to the apparent wind are attempted. This necessity of operating at large angles to the wind has a large number of disadvantages. The obvious one is the necessity of tacking at 40.degree.-45.degree. to the true wind, which means that even finely tuned sailboats can only make progress in the windward direction in 86.degree.-90.degree. increments. This disadvantage is increased in fore-and-aft rigged vessels, i.e., vessels having more than one mast, typically having a mizzen mast bearing one sail rigged on a boom behind that mast generally in the manner of a mainsail (e.g., a ketch or a yawl). Partially because of the effect of the forward sails on the wind before it reaches the aft sail(s), fore-and-aft rigged sailing craft are generally even less able to sail effectively at close angles to the wind than the simple sloop rig.
Another substantial disadvantage is the almost immediate loss of power and forward force which a standard sailboat experiences at angles closer into the wind than its optimum angle. As the angle into the wind decreases, the forward thrust drops, then the luff or forward edge of the sail begins to flap because of the air turbulence caused by the improper angle of the leading sail edge to the wind (often termed "luffing"), and then the sail loses its shape, flaps in the wind across its width, at which time it has lost its forward thrust entirely and the wind force on the flapping sail becomes almost purely a dragging force, tending to push the vessel backward, rather than forward. Because of the strong desire to point as closely into the wind as possible to reduce the distance the vessel must travel, sailors, particularly inexperienced ones, tend to sail at a closer angle into the wind than they should, somewhere between the optimum angle of the rig and the point where the sails are luffing. Since the sails appear full, and the apparent wind feels strong, the prime symptoms of sailing at such angles is the loss of power, which is difficult for the inexperienced to detect until it is too late, and headway is lost.
The inability to develop power at close angles to the wind is also one of the prime factors which has reduced the useability of sail power in commercial freight and passenger transportation. Faced with the prospect of having to veer forty-five or more degrees off course in order to obtain benefit from sail power, shippers do not make the capital investment to rig their vessels, but rather choose to expend the fuel costs and take the direct route under power.
Accordingly, it is an object of the present invention to provide a sail system which offers substantial forward thrust even at close angles to the wind.
It is another object of the invention to provide a sail system which has a wide tolerance of sailing efficiency, i.e., it produces substantial forward thrust without substantial adjustment over a wide range of angles to the wind.
It is a particular object of the present invention to provide a sail rigging system which permits fore-and-aft rigged vessels to sail efficiently at smaller angles into the wind.
It is a further object of the present invention to provide a sail rigging system which provides an improved ratio of driving force to heeling force over a wide range of angles to the wind.
It is a further object of the invention to provide a sail rigging system which is easily operated with minimum manpower over wide ranges of wind speed, particularly over the range of medium to heavy winds.
These and other objects and advantages which will be apparent to the skilled in the art from a consideration of this disclosure or practice of the invention disclosed herein are achieved by the invention described in greater detail below.